The purpose of this study was to investigate how the recruitment gain, as derived from the Hoffmann (H)-reflex measurement, of antagonistic motoneurons is modulated during voluntary lengthening (LEN) and shortening (SHO) contractions in the human tibialis anterior (TA) muscle. To this end, the parameters of the ratios of the slope in each ascending part of the H and M recruitment curves (Hslp/Mslp) in the antagonist muscle were compared between LEN and SHO contractions in nine young, healthy subjects. Although there were no differences in the levels of background activity (BGA) between LEN and SHO contractions in the agonist (TA) and antagonist (soleus) muscles, the Hslp/Mslp of the antagonist muscle was significantly higher during LEN contractions than during SHO contractions (P<0.01). It was therefore demonstrated that the recruitment gain of the antagonistic motoneurons to the la afferent inputs was higher during LEN contractions than during SHO contractions despite similar BGA levels. This result might reflect differences in the extent of the reciprocal inhibition from the agonist to the antagonist muscles and/or in the neural mechanism underlying the length-changing manners of the antagonist muscle itself.